Wouldn't help you with your back to the tree, but I killed two does this past weekend at 140 and 120 yards respectively using the Tethrd HYS Strap and two of the new Fold n Go hangers. Worked great as a rifle rest and both does bang flopped. Most of my saddle rifle shots have been within 75 yards and generally free hand, so I was thrilled with how good of a rest I was able to get with this set up leaning into the tree in a sitting position.
I think going with the rope set up as mentioned above would work well for a traditional hang on, or going with a monopod/tripod like the Primos trigger stick. Practicing free hand shots would be beneficial as well.
Killed a lot of deer with my left elbow propped on my left knee too.